In DE-A 37 06 853, a process is disclosed wherein inter alia potassium heptafluorotantalate is introduced into an inert salt melt at a temperature above 600.degree. C., earth acid metals are obtained metallothermically from complex halides of the earth acid metals by reduction with alkali or alkaline earth metals at temperatures at which the salts involved in the reaction are present in molten form.
According to DE-A 37 06 853, the exothermic reduction reaction results in heating of the reaction mixture to beyond 700.degree. C. Thereafter, the reaction is sustained for another 4.5 hours at 850.degree. C., the salt melt being stirred together with the tantalum sediment.
In one particular Example of this patent specification, the addition of alkali metal and tantalum salts is spread over several individual steps and the final temperature is kept at 730.degree. C.
In this process, the tantalum salt and the reduction metal are generally introduced continuously into the reactor in partial quantities to control the temperature conditions prevailing during the exothermic reaction of the components.
One feature common to the state-of-the-art processes is that the starting material is generally a salt melt which, to control the highly exothermic reaction, mostly contains inert diluting salts and in which the tantalum salt (potassium heptafluorotantalate) is always present in molten form before the reduction metal is introduced.
In every case, the reaction temperatures are at least above 600.degree. C. and the stirred salt melt is always kept at elevated temperature for a certain time to guarantee complete reaction of the reactants. However, although another requirement is that the maximum temperature of the reaction mixture be kept as low as possible, the resulting end products always consist--after cooling of the salt melts--of compact material in the form of solid alkali salt lumps in which the metal powders are unevenly distributed and which always have to be size-reduced by crushing and grinding before they are subjected to washing.
Accordingly, the state-of-the-art processes for the production of valve metal powders have various disadvantages of which the following are particularly important in connection with the problem addressed by the present invention:
The fusion of the components (inert salts and earth acid metal complex salts) requires temperatures which in turn necessitate heat-resistant and corrosion-resistant reactor materials. The stirring of the melt naturally leads to abrasion of the crucible wall material and to contact between the tantalum powder formed and the reactor wall, resulting in harmful metallic impurities in the product from the crucible material. In addition, the metal powder particles are compacted by thermal and mechanical effects (forging effects) in a stirred salt melt and, as a result, undergo unwanted surface losses.
State-of-the-art processes in which the salt melt is not mechanically moved suffer from the drawback of an incomplete reaction of the reactants and require a large excess of reducing agents or correspondingly long residence times at high temperatures.
Accordingly, extremely pure earth acid metal powders require high operational outlay for their production, for example through the use of inlets of special metal sheets inside the reactor. However, these inlets have only limited useful lives and, in addition, are extremely expensive.
The equally well-known "paste processes", for example according to DE-C 25 17 180, in which the reactants are reacted in the form of a paste-form mixture by initial ignition and have to be kept at a relatively high temperature for a while without stirring, lead to spontaneous overheating of the reaction mixture on ignition, accompanied by fusion of the salts involved. The reaction kinetics cannot be controlled. As mentioned above, the long residence times of the crude powder in the hot salt melt again lead to coarse tantalum powders of which the means grain sizes, as measured by the air penetration method (FISHER SUB-SIEVE SIZER, ASTM B 330/82), exceed values of 1 micrometer in every case.
Accordingly, the object of the present invention is to provide high-purity earth acid metal powders which have a relatively small mean grain size without any of the disadvantages of the metal powders produced by known processes for capacitor applications.